The visitors took some time to wake up in front of the Neath crowd and, after eight minutes, slack defending from a Henson chip and Shane Williams hack on allowed young scrum-half Rees to outpace Romain Froment in the corner for a converted Ospreys try.

A big forward drive won a penalty - kicked by Marticorena - to settle the French, but Henson cancelled that out with a fine goal kick into the stiff wind.

We've always said that next year's campaign started with this game

Ospreys assistant coach Sean Holley

Castres' Scotland full-back Glenn Metcalfe then sparked an attack, carried on dynamically by the French tight forwards.

The ball came to tight-head prop Galasso, who steamed through the middle of the disorganised defence for a bullying score.

With Castres' pressure mounting, Rees was sin-binned for an off-the-ball obstruction, and Marticorena's resultant penalty gave the visitors an 11-10 lead at the break.

Richie Rees scored a try but also visited the sin bin

But the centre had badly scuffed two other shots, and continued the poor place-kicking form in the second half.

With the wind at his back, Henson landed a shot at goal to take his team into a two-point lead.

The home side held the slim advantage into injury time despite the mighty effort of the intimidating French forwards, Ospreys flanker Steve Tandy impressive in the rearguard action.

With the game slipping away, the in-form Seveali'i broke away after intercepting a Ugo Mola pass for the sealing try with the last move of the match.

It was a messy game, we were too
hurried in our passing and our kicking

Castres coach Christophe Urios

Ospreys assistant coach Sean Holley told BBC Sport Wales.

"Castres asked a lot of questions up front and had dangerous runners behind. They threw everything at us, but we dealt with it."

Castres coach Christophe Urios said: "It was a messy game, we were too
hurried in our passing and our kicking."